1. Field of the Invention
The invention relates to a universal serial bus (USB) transmission system, and in particular to a universal serial bus transmission system which can prevent high-speed transmission signals from electromagnetic interference (EMI) by using a low-speed transmission interface to transmit a control signal and a data signal, thereby reducing costs for handling electromagnetic interference.
2. Description of the Related Art
Since IBM company developed personal computers (PCs), PC XT and PC AT, IBM PCs and compatibles have been widely used in the whole world. During this period the functions of PCs are greatly enhanced, and more peripherals, such as GUI operating systems and mouse, arc provided in line with continuous progress in the semiconductor technology. To print required data out, printers seem to become a necessary peripheral. Moreover. moderns are widely used by users to connect popular internet. To avoid a large amount of data from losing, resulting from breakdown of computers back-up devices with a large amount of storage capacity are prepared for regular data back-up. Scanners are also a widely-used peripheral for scanning graphics.
So far, all peripherals stated above have been widely, even necessarily, used. However, each peripheral has been developed with a different purpose in a different circumstance. Therefore, a different interface is designed for each peripheral. For example, a RS232 serial interface is used for mouse, and a parallel interface is used for printer. Additionally, some interfaces need to install additional interface cards. For example, a small computer system interface (SCSI) used to connect back-up devices with a large amount of storage capacity needs an additional interface card installed. Particularly, some peripherals use their own specific interfaces. For example, a number of scanners use a special interface which needs a specific interface card installed. Thus, a personal computer must provide a number of slots for installing various interface cards. If a personal computer simultaneously uses all the peripherals described above, relatively more connective lines used to electrically connect the personal computer and the peripherals are needed, resulting in a mess.
To overcome the above-stated disadvantages, a specification of USB interface has been developed, and allows all peripherals to use the standard interface for connection. As a result, the installation for a personal computer system is greatly simplified, and it is unnecessary to leave an excessive space for installing special interface cards. In addition, the USB interface has a PnP function. That is, a personal computer can automatically detect all connected peripherals, and install related drivers for users. For a further clear description, the USB interface has a play-and-plug function, and can connect installed peripherals during the operation of a personal computer. After that, the personal computer can automatically detect all the connected peripherals, and install related programs without shutting down and then rebooting the personal computer. Thus, users can freely use all the connected peripherals. Similarly, when the peripherals are disconnected, the personal computer can automatically detect this status, then removes corresponding programs, thereby avoiding the users from using the corresponding programs to cause operational errors.
Even though the USB interface has a number of advantages mentioned above the USB interface are easily interfered by high-voltage regions or generates electromagnetic fields to interferes other surrounding devices, resulting from the high transmission rate thereof. As an example, in a simple USB monitor system, the USB interface is integrated in a monitor controller IC.
FIG. 1 is a schematic view showing a conventional USB transmission system 10. In FIG. 1, a monitor controller IC 12 is disposed on the front end of a monitor system 10. The monitor controller IC 12 is electrically connected to a plug 14 through a cable 16. The cable 16 passes through a number of high-voltage regions 18. Currently, the USB interface has a high transmission rate of approximately 1.5M or 12M bits/sec. During the transmission, the USB interface is easily interfered by the high-voltage regions 18, and even interferes other surrounding devices.
Referring to FIG. 1, an electromagnetic protective region 19 is formed surrounding the cable 16, thereby preventing signals transmitted through the cable 16 from being interfered by electromagnetic fields. However, This increases costs for handling electromagnetic interference (EMI) of the monitor system.